This invention pertains to an improved process and catalyst for the hydro-oxidation of olefins, such as propylene, by oxygen in the presence of hydrogen to form olefin oxides, such as propylene oxide.
Olefin oxides, such as propylene oxide, are used to alkoxylate alcohols to form polyether polyols, which find wide-spread utility in the manufacture of polyurethanes and synthetic elastomers. Olefin oxides are also important intermediates in the manufacture of alkylene glycols, such as propylene glycol, and alkanolamines, such as isopropanolamine, which are useful as solvents and surfactants.
The direct oxidation of olefins having three or more carbon atoms (C3+ olefins) with oxygen has been the subject of intense industrial interest for several decades. Much effort has focused on the direct oxidation of propylene by oxygen to propylene oxide. Such a process is sought for replacement of indirect multi-step manufacturing processes currently in practice, including the well-known chlorohydrin and organic hydroperoxide routes to propylene oxide. It is known that silver catalysts can catalyze the direct oxidation of propylene with oxygen to propylene oxide in a selectivity of not more than about 70 mole percent. Disadvantageously, the process produces significant quantities of partial oxidation by-products, including acrolein, acetone, and propionaldehyde, as well as deep oxidation by-products, namely, carbon monoxide and carbon dioxide.
Over the past ten years many patents have disclosed direct hydro-oxidation of olefins having three or more carbon atoms with oxygen in the presence of hydrogen to form olefin oxides. Catalysts for the hydro-oxidation are disclosed to comprise gold, silver, and noble metals, such as palladium and platinum, and optionally one or more promoters, such as alkali, alkaline earths, and rare earths, deposited on a titanium-containing support, such as, titania or a titanosilicate zeolite. In particular, gold or gold in combination with silver and/or a noble metal (e.g., bimetallic catalyst with palladium) has been the subject of considerable patent activity, with some patents disclosing oxidized gold as a catalytically-active species and other patents disclosing metallic gold of a particle size greater than 1 nanometer (nm) and less than about 100 nm as the catalytically-active species. A representative group of patents drawn towards the hydro-oxidation of C3+ olefins using catalysts comprising gold, silver, and/or a noble metal deposited on a titanium-containing support include the following: EP-A1-0,709,360, WO 98/00413, WO 98/00414, WO 98/00415, U.S. Pat. No. 6,255,499, WO 03/062196, WO 96/02323, WO 97/25143, and WO 97/47386. In the aforementioned prior art, the catalysts are prepared by impregnation of or precipitation from one or more solutions of soluble salts of gold, silver, and/or one or more noble metals and soluble salts of one or more promoters. The aforementioned prior art disclose for such hydro-oxidation processes a high selectivity to C3+ olefin oxides, most particularly, propylene oxide. A propylene oxide selectivity greater than about 90 mole percent is achievable; and selectivities to propylene oxide in excess of 95 mole percent have also been reported.
Despite such advances, several problems of the prior art need to be addressed before the hydro-oxidation route can replace current manufacturing processes for preparing olefin oxides. First, hydrogen efficiency needs to be improved. Hydrogen is a necessary reactant in producing the olefin oxide. For every mole of olefin oxide produced, the olefin hydro-oxidation produces a stoichiometric equivalent of water. Additional water can also be formed through one or more undesirable side-reactions, for example, by the direct oxidation of hydrogen with oxygen. Hydrogen efficiency can be ascertained by measuring a molar ratio of water to olefin oxide in the product stream, e.g., water to propylene oxide (H2O/PO). Desirably, the ratio is 1/1; but in practice, at any specific time during process operation, a higher ratio is usually observed. Moreover, with current prior art catalysts, the formation of water and the water/olefin oxide molar ratio increase unacceptably with time. While it is informative to track the water/olefin oxide molar ratio at intervals throughout a process, a cumulative water/olefin oxide molar ratio may be more indicative of overall hydrogen efficiency. For the purposes of this invention, the term “cumulative water/olefin oxide molar ratio” means the average water/olefin oxide molar ratio over the total run time, preferably, averaged from measurements of the water and olefin oxide concentrations in the product stream taken at least every three hours, preferably, at least every two hours, and more preferably, every hour. In prior art processes over time, the cumulative water/olefin oxide molar ratio increases and often exceeds greater than about 10/1, which is unacceptably high.
Second, prior art processes operate at a temperature typically between about 70° C. and about 170° C. Beyond this temperature range, and often within this range depending upon the catalyst, prior art processes exhibit decreased selectivity to olefin oxides and increased selectivities to undesirable partial oxidation products (e.g., propionaldehyde, acetone, acrolein), deep oxidation products (namely, carbon monoxide and carbon dioxide), hydrogenation products (e.g., propane), and water. Moreover, prior art catalysts tend to deactivate quickly with increasing temperature. Operating at higher temperatures, for example, at 160° C. or higher, with stable activity and selectivity is desirable, because a hotter water co-product (steam) can be utilized, if desired, in down-stream plant operations. Heat integration resulting therefrom can be beneficial to overall plant economics and management.
Third, in determinations of the overall economics of hydro-oxidation processes, the quantity of gold, silver, and noble metal in the catalyst should be taken into consideration. Gold, silver, and noble metals are notoriously expensive; therefore, any decrease in the quantities thereof required for the hydro-oxidation catalyst would provide added value.
Fourth and most importantly, prior art hydro-oxidation catalysts exhibit decreasing activity over time and reach a reduced level of activity after several days. At such time, the hydro-oxidation process must be shut down, and the catalyst must be regenerated. A need exists in the art to stabilize the activity of the catalyst over a longer run-time, so as to increase the intervals between catalyst regenerations and to increase overall catalyst lifetime. The term “catalyst lifetime” as used herein refers to the time measured from the start of the hydro-oxidation process to a point at which the catalyst, after one or more regenerations, has lost sufficient activity so as to render the catalyst unacceptable, particularly, from a commercial point of view.
We note that T. Alexander Nijhuis, et al. in Industrial Engineering and Chemical Research, 38 (1999), 884-891, discloses a catalyst containing gold particles on the exterior surface of a titanosilicate support for the hydro-oxidation of propylene with oxygen in the presence of hydrogen to form propylene oxide. The catalyst is prepared by conventional deposition-precipitation from an aqueous solution of gold (III) chloride.
Further, several references, as illustrated by T. V. Choudhary, et al., Journal of Catalysis, 207, 247-255 (2002), disclose nano-gold catalysts supported on titania prepared from gold-phosphine ligand cluster complexes. WO 2005/030382 discloses a heterogeneous catalyst comprising gold particles on a support medium, such as titanium oxide-coated alumina, wherein the gold particles are physically vapor deposited at a Penetration Depth Ratio in a range from about 1×10−9 to about 0.1. These references are silent with respect to hydro-oxidation processes.